Electrically augmented vacuum processes and apparatuses for carrying out dewatering of suspensions are disclosed in U.S. Pat. No. 4,107,026, issued Aug. 15, 1978, U.S. Pat. No. 4,168,222, issued Sept. 18, 1979, U.S. Pat. No. 4,170,529, issued Oct. 9, 1979, U.S. Pat. No. 4,207,158, issued June 10, 1980 and copending applications, U.S. Ser. No. 109,931, filed Jan. 4, 1980 and U.S. Ser. No. 285,196, filed July 20, 1981. The entire disclosures of these patents and applications are incorporated herein by reference thereto.
In these earlier disclosures, self-contained hollow electrode assemblies are provided which are normally submerged in the suspension, but allow bodily removal therefrom for inspection or other purposes. These hollow electrode assemblies comprise two types of wall surfaces: ion-pervious walls for electrode assemblies of one polarity and liquid-pervious walls for electrode assemblies of the opposite polarity. The wall surfaces of the electrode assemblies comprise chemically and electrically neutral filter media, an ion exchange membrane or a permeable porous membrane, backed by a supporting grid, thus presenting a planar electrode surface.
In operation, with electrode assemblies of both types immersed in the suspension, a source of vacuum is connected to the interior of the electrode assemblies having liquid-pervious walls to provide a controllable pressure differential, thereby producing a flow of carrier liquid through the filtration surfaces, while the solids migrate in the opposite direction, under the influence of the electric field, to deposit as cake upon the electrode assemblies having ion-pervious walls. Filtrate or permeate liquid, that is, carrier liquid freed of solids, is withdrawn or pumped from the interior of the liquid-filled hollow electrode structure at a controllable rate.
As indicated, cake deposition occurs on the hollow electrode assemblies having ion-pervious surfaces; these electrode assemblies are filled with an electrolyte and have an electrode element therein immersed in the electrolyte. These electrode elements are thus isolated from direct contact with the suspension. The electrolyte is specially selected for high conductivity and compatibility with the electrode element. By compatibility is meant the relatively non-corrosive character of the electrolyte relative to the electrode element under the conditions that ordinarily prevail within the hollow electrode assembly. Since decomposition or evolution products and heat are generated at the electrode element within the hollow ion-pervious assembly, provision is made for a flow of electrolyte into and through the electrode chamber so that foreign products, including gases, and heat are swept out of the chamber and a relatively constant predetermined electrolyte composition is maintained.
The ion-pervious wall of the electrode assembly in these prior art structures comprises a chemically and electrically neutral filter medium, ion exchange membrane, or permeable porous membrane which, if film-like in nature or otherwise requiring support, may be backed by a chemically and electrically neutral grid so that a planar electrode wall surface is presented to the slurry being treated. Since the cake will form on this electrode during electrofiltration and must be removed by contact with doctoring blades, a friction cage or spacing means may be provided to protect the relatively fragile wall surface from direct contact with the doctoring blades. The friction cage comprises a thin, open screenwork of relatively hard material covering the wall surface of the electrode assembly for contact with the doctoring blades while spacing means may comprise strips of plastic materials, such as Delrin acetal resin arranged in a frame-like configuration and of a thickness sufficient to prevent contact between the doctor blade and the wall surface. For the purpose of cake recovery, the electrode assembly is raised to a position of emergence from the suspension, with the layer of collected solids or cake layer adhering thereto. Since the electrolyte remains within the electrode assembly in the raised position, a vacuum is applied internally of the electrode to reduce the pressure and so prevent rupture or distortion of the assembly walls. When the electrode assembly is immersed in operation, the vacuum applied internally serves to help remove gaseous products, such as chlorine, oxygen or carbon dioxide, evolved at the electrode element.
As described above, a plurality of electrode assemblies are immersed in a bath, arranged so that electrode assemblies of one polarity alternate with electrode assemblies of the opposite polarity; electrode assemblies of like polarity being electrically connected in parallel. With this arrangement a plurality of anodes and cathodes with their electrical connections must be provided and special and expensive structures such as electrode elements and equipment for circulating electrolyte must be provided for each such anode or cathode.
Accordingly, a need exists for an electrofiltration apparatus which does not require the multiple, complex structures of the present commercial apparatus.